1. Field of the Invention
The present invention relates to a driving method and a driving circuit of a light source apparatus.
2. Description of the Related Art
In order to implement high density of an optical disc and high fineness of a display, a small-size short wavelength light source is needed. As the small-size short wavelength light source, a coherent light source has attracted attention (Yamamoto et al., Optics Letters Vol. 16, No. 15, 1156 (1991)), which combines a semiconductor laser with a light waveguide type of second harmonic generation (referred to as “SHG” hereinafter) device coupled by a quasi-phase matching (referred to as “QPM” hereinafter) method (light waveguide type of QPM-SHG device).
FIG. 1 is a schematic constitutional view of a SHG blue light source using the light waveguide type of QPM-SHG device. As shown in FIG. 1, as a semiconductor laser, a wavelength tunable DBR semiconductor laser 54 having a distributed Bragg reflector (referred to as “DBR” hereinafter) region is used. The wavelength tunable DBR semiconductor laser 54 is a 100 mW category AlGaAs group wavelength tunable DBR semiconductor laser having 0.85 μm band, and comprises an active layer region 56, a phase adjustment region 57 and a DBR region 58. An oscillation wavelength can be continuously varied by controlling a current input to the phase adjustment region 57 and the DBR region 58 at a constant ratio.
A light waveguide type of QPM-SHG device 55 which is a second harmonic generation device consists of a light waveguide 60 and a cyclic polarization reversal region 61 formed on an X plate of MgO doped LiNbO3 substrate 59. The light waveguide 60 is formed by proton exchange in pyrophoric acid. The cyclic polarization reversal region 61 is formed by forming a comb-shaped electrode on the X plate of MgO doped LiNbO3 substrate 59 and applying an electric field thereto.
According to the SHG blue light source shown in FIG. 1, laser of 75 mW is input to the light waveguide 60 from a laser output of 100 mW. An oscillation wavelength is fixed within a phase matching wavelength tolerance width of the light waveguide type of QPM-SHG device 55, by controlling a current amount input to the phase adjustment region 57 and the DBR region 58 in the wavelength tunable DBR semiconductor laser 54. In this SHB blue light source, blue light of about 24 mW having a wavelength of 425 nm is provided. The provided blue light has diffraction-limited converging characteristics in which a transverse mode is TEoo mode, and noise characteristics in which a relative noise intensity is small, that is, not more than −140 dB/Hz, that is, this has the characteristics suitable for reproducing the optical disc.
Meanwhile, according to the light waveguide type of QPM-SHG device 55 serving as the second harmonic generation device, in view of the blue light output characteristics to the wavelength of a fundamental wave light, a wavelength width in which its blue light output becomes half, that is, a wavelength tolerance width to the phase matching is as small as 0.1 mm. This is a big problem for getting a stable blue light output. In order to solve this problem, conventionally, a wavelength (oscillation wavelength) of the fundamental wave light emitted from the wavelength tunable DBR semiconductor laser 54 is fixed within the tolerance width of the phase matching wavelength of the light waveguide type of QPM-SHG device 55 so as to implement the stable blue light output.
In general, the oscillation wavelength of the semiconductor laser is varied by ambient temperature, and an optimal wavelength of the light waveguide type of QPM-SHG device 55 is also varied by the ambient temperature. Therefore, conventionally, the blue light output is stabilized by keeping the temperature of the semiconductor laser 54 and the light waveguide type of QPM-SHG device 55 constant using a Peltier device and the like.
However, when it is mounted on an optical information processing apparatus such as an optical disc or a laser printer, an average output power is continuously varied in an activated state. At this time, an amount of heat generated by the semiconductor laser is varied. Therefore, even when the ambient temperature is kept constant by the Peltier device and the like, the temperature of the semiconductor laser itself is varied so that the oscillation wavelength is varied. As a result, the stable blue light output cannot be provided.
In addition, when the temperature controller such as the Peltier device cannot be used because of the miniaturization of the apparatus, the ambient temperature is further largely varied, which causes an output variation of the light waveguide type of QPM-SHG device 55.
Furthermore, in the optical disc device, for example, a temperature of the active layer region 56 is varied by a current variation input to the active layer region 56 at the time of high-speed modulation and an effective optical distance L of the wavelength tunable DBR semiconductor laser 54 is varied. In order to solve the problem, in the prior art, as shown in FIG. 2A, the heat amount of the whole of the wavelength tunable DBR semiconductor laser 54 is kept almost constant by applying a current which is complementary to the current (driving current) input to the active layer region 56 as shown in FIG. 2A, to the phase adjustment region 57 as shown in FIG. 2B (hereinafter, this driving method is called a “complementary compensation method”). At this time, since the optical distance in the active layer region 56 and the optical distance in the phase adjustment region 57 are varied almost symmetrically, the effective optical distance L of the wavelength tunable DBR semiconductor laser 54 can be kept constant. Therefore, the oscillation wavelength of the wavelength tunable DBR semiconductor laser 54 can be controlled and the variation of the blue light output power can be prevented (Japanese Laid-open Patent Publication No. 2001-326418).
However, there is a problem in the above complementary compensation method. When the current is input to the phase adjustment region 57, an electric charge density in the phase adjustment region 57 is varied in response to the current, whereby a phenomenon in which a refractive index in the phase adjustment region 57 is varied (this phenomenon is called a “plasma effect”). Therefore, when the current complementary to the current (driving current) input to the active layer region 56 is applied to the phase adjustment region 57 in order to keep the heat amount in the whole of the wavelength tunable DBR semiconductor laser 54 almost constant, the effective optical distance of the semiconductor laser is varied because of the plasma effect and the blue light output power is varied.